Method of producing high tensile strength iron



Patented Dec. 10, 1940 UNITED STATES METHOD or PRODUCING HIGH TENSILE STRENGTH IRON James L. Gibney, Bufialo, N. Y.

No Drawing. Application August 14, 1937, Serial No. 159,195

7 Claims.

This invention relates to the production of high tensile strength iron having novel physical properties in the untreated cast state and more particularly to the production of hypereutectoid iron having a total carbon content of from .90

. This application is a continuation in part of my copending patent for Method of producing high tensile strength iron, No. 2,169,464, dated August 15, 1939.

In general the invention relates to the control of the prenatal condition of the parent austenite in the production of .90 to 2.00% carbon iron to provide, in the cast state, an exceptionally small [5 grain size and freedom from pronounced dendritic patterns and also to the production of iron, having in the cast state, precipitated temper carbon in the form of small graphitic nodules which are I unafiected by subsequent air, oil or water quenchj ing, and the precipitation of which produces, on

final cooling in the cast state, a greater proportion of pearlitic structure than an iron not having such precipitated carbon.

, 35 uct. The temper carbon is precipitated in nodular form by introducing at least two graphitizing agents selected from a group comprising silicon, titanium, nickel and aluminum to the blended metals although temper carbon can be so precipitated from hypereutectoid iron produced by methods other than by the blending of high and low carbon content irons, as herein described. The product forming the subject of this invention has without alloying remarkable physicals both 45 in the cast and heat treated state, having high tensile and torsion strength with high angular deflection, accompanied by high elastic limit, impact and high modulus of elasticity. Further, the process can be readily controlled to render 60 the castings machinable in the cast state without subsequent heat treating and the castings readilyrespond to torch or flame hardening, the temper carbon nodules, during torch hardening, producing a tough martensitic wear resisting sur- 55 face because of the lubrication of the surface by The fine grain structure, in the cast state, is produced by prethe small evenly divided graphite particles. The

usual carbide forming or hardening alloys, such as chromium, molybdenum, tungsten and vanadium, can also be incorporated.

The principal object of the invention, as pre- 5 viously indicated, is to produce, by a prenatal control of the parent austenite, a hypereutectoid iron having in the cast state extremely fine grain structure, thereby providing, without the necessity of special alloys or mechanical working, a 10 metal capable of having high tenacity and torsional strength with high angular deflection, accompanied by high elastic limit, impact and high modulus of elasticity.

1 Another object is to produce, by a prenatal 5 control of the parent austenite, hypereutectoid iron having a. very fine or diminished dendritic pattern in contradistinction to the coarse dendrites usually produced upon primary solidification in accordance with conventional steel making 20 practice. V

Another object is to provide a process in which the desired grain structure is obtained directly in the cast state thereby to eliminate the necessity of a long cycle of subsequent heat treating operations to effect a modification of the cast structure.

Another object is to provide a material which can be readily heat treated in sections above one- .half inch size, such larger sections of the present material readily responding to secondary heat treatments such as are employed in steel making whereas with the heat treatment of white,iro'n, the sectional dimensions are necessarily restricted to from A to /2 inch to insure uniformity and complete softness in the final product during the malleab-leizing cycle.

Another object is to produce a hypereutectoid iron in which temper carbon is precipitated from the solution during the process of solidification 40 and consequent cooling to provide extremely small, round or slightly elongated temper carbon nodules in the solidified castingwhich are not materially altered, deformed or ref rmed by air,

oil or water quenching but may lie substantially altered by means of secondary'f 'nace coolings.

Another object is to produce hyp'ereutectoid iron in which the temper car on so precipitated forms, in the solidified cast product, a greater proportion of pearlitic structure than a product having no precipitated carbon.

Another object of the invention is to produce hypereutectoid iron which has a decreased volumetric shrinkage, thereby producing a high yield, by weight, of castings per pound of metal melted and an extremely rapid setting product which tends to provide'a more solid casting material by gaseous fuels or electric arcs and are main-- tained at such high temperatures during the socalled refining periods following the reduction of metalloids in the furnace. The metal poured from the furnace is therefore at a high temperature, this being usually in excess of 2800 F. and sometimes being as high as 3200 F. I have found that the period of cooling from such high steel making temperatures to .the liquidus or start of solidification point has a very pronounced prenatal effect on the parent austenite which starts to form on reaching the liquidus point. I have -further found that, irons of .902.00% carbon content can be made at much lower temperature by following the process outline in my said copending application, and that with the resultant lessened cooling period to the liquidus point, the product has, in the cast state, an extremely small grain size and a very diminished dendritic pattern as compared with castings made from metal refined at the usual high steel making temperatures above indicated.

The micro-structure of conventional steels,

whether straight carbon or alloyed, produced at the. usual high steel making temperatures above indicated, are readily diagnosed at 100 or normal magnification and follow the regular ingot patterns. The consistently large grained austenitic formation-produced in steels made at the usual high steel making'temperatures have come to be regarded by steel makers and metallurgists as an expected and inevitable phenomena of steel making and, so far as I amaware, no attempt has been made to exercise a prenatal control over the metal to reduce the size of the grain structure in the product of primary solidification and cooling. To eliminate the coarse grained structure of the conventional cast product the steel making industry depends on forging and breaking down of the coarse crystallite formation'and on subsequent careful treating to restore high physical characteristics.

It is generallybelieved that the growth of a crystal of primary austenite starts to form at the liquiclus point, which with 1.50% carbon steel is approximately 2560 and increases in size on undisturbed slow cooling down to its solidification point or approximately 2090 F. With the long sojourn of cooling from the high steel making temperatures of, say 3182 F. generally employed, down to the solidification point, the,

austenite grows to a large size and the microstructure of the solidified product shows large pearlitic grains usually of octohedral or polygonal form with excess cementite segregated on the grain boundaries, these pearlitic grains in quantity being between 3 to grains to the area exposed under the microscope at 100 magnifications, this area being usually of an inch in size. The slow cooling from 3182 F. down to 2560 F. where solidification starts, sets up a. re-

tardation in'the selective solidification so that on' further cooling heavy, coarse ingot patterns are caused to form. Further, the slow solidification of the liquid metal produced at such high temperatures causes a precipitation of the low melting point elements resulting in a lack of structural homogeneity and rendering the product diflicult to heat treat.

The present invention also comprehends a new form of temper carbon. In conventional steel making practice in foundries, this coarse ingot grain structure is slowly broken up within and around the grain proper by long soaking at high temperatures of from 1650 to 1800 F. this operation being known as "normal-hing followed by cooling in air or in a furnace to render the steel tough and usable. In the manufacture of high carbon steel such normalizing or secondary heating and slow cooling throws out of solution small percentages of graphitic carbon similar to the type and dispersion found in malleable iron. This temper carbon formation is produced entirely by reheating and such steel heretofore has been considered unusable.

Malleable iron is first produced as a white iron withall its carbon in theform of combined carbon and is then rendered soft and malleable by,

with other processing. With all malleable irons the temper carbons are formed by dissociation of the iron carbide or cementite into temper carbon nodules by a long time' treatment at high ternperatures and a subsequent slow cooling operation. The carbon formed is rosette shaped-a rounded or ragged form of temper carbon of large size and easily detected by the naked eye and is approximately 3 to 6 times larger than the temper carbon nodules forming the subject of the present invention.

.In pearlitic malleables, the structure is similar to malleable iron, the temper carbon nodules being contained in a matrix of fine ferrite grains and the pearlite may bedisseminated throughout the matrix by low temperature drawing, annealing or spheroidizing. The pearlite formed by interrupting the annealing cycle upon being heated in the spheroidizing ranges coalesces into fine spheroids, usually breaking up all form oflamellar pearlitic formations. These struc- Y tures are found in irons within the carbon ranges of 2.00 to 2.80% which were white, hard and brittle in the cast state. To render the castings a melt of high carbon iron (2-3.5% carbon) at a predetermined low temperature of from 2300-2800 F. This metal is mixed with molten low carbon partially finished straight steel or partially finished alloyed steel'(.10 to 1.50% carbon) having an extremely high temperature of from 2800 to 3200" F. in any proportion to provide a low temperature casting metal of approximately 2750 F. having any desired carbon content within the hypereutectoid range of .90 to pattern and a finer grain structure in the cast This grain size is approximately one-' '10 tenth to one-twentieth the size of the usual in- 2.00% carbon.- The amalgamation of these two unlike heat of solution metals is endothermic and hence the temperature of the mixed metals is less than the mean temperature of the high and low carbon metals before mixing. As a consequence,

when the mixed metals are poured the castings produced have a greatly diminished dendritic state.

got grain whch is produced by conventional furnace methods of making iron or steel and is about equivalent to the grain size produced in a chilled cast product. Themetal made and cast in ac cordance with my invention greatly resembles steel that has been broken down by heat and forging or otherwise worked and then treated. The fine grain structure with absence of pronounced dendritic pattern is produced by the low pouring temperature immediately following the blending, no holding period being necessary beforeteeming and no subsequent periods of refining being required. Further the production of small grain size during solidification produces'a rapid rate of setting which results in a lower volumetric shrinkage, and less danger from cracking or hot tears. This product can be used as cast due to its small grain sizeand structural uniformity and can also be treated to develop high physical values.

In carrying out the above process in producing a straight carbon analysis iron having no alloys it was observed that upon cooling the metal had small grains as cast resembling somewhat a hot rolled steel. The grains were small and. had a characteristic iron carbide (cementite) grain boundary. Upon cooling the iron carbide segregated on the grain boundary proper and high magnification showed its of! branches or oil shoots radiating from the grain boundary into the grain proper. Under high magnification the carbide within the grain was distinctly indicated. The grain size was one-tenth to one-twentieth the size of the usual ingot grain produced by slow cooling, such as is the case with conventional slag processes or steel making procedure. In making a grain size comparison with well known existing standards of the A. S. T. M. or Timken index numbers, the present cast state grain is equivalent to a '7 grain produced by redissolving the micro-constituents above its upper critical points, AC3 or ACM, and subsequently cooling by various methods oi! cooling, this being the standard method of checking grain size by the industry. The prenatal control of the parent austenite accounts for this tremendous decrease in initial grain size.

I have further discovered that by introducing; a combination of at least two graphitizing agents in irons of the .90 to 2.00% carbon range, preferably, but not necessarily, produced by the above process ,of mixing low temperature high carbon iron with high temperature steel, carbon, during the process of solidification, is precipitated as a new form of temper carbon. The graphitizing agents which may be employed for this purpose are silicon, titanium, nickel and aluminum. The temper carbonso precipitated is in the form of fine, nodules which are rounded or slightly elongated and of a size approximating one-third to one-sixth the size of temper carbon formations developed in malleable iron or pearlitic malleables by secondary slow cooling or by interrupting the cooling cycle as previously described.

The new type of temper carbon resulting from the practice of my invention segregates uniformly throughout the structure of the product and is not substantially altered in size by subsequent air, water or oil quenchings but can be substantially altered by means of secondary furnace coolings. precipitated out of solution as nodular temper carbon is preferably .02% or more and may be regulated to throw out as much as 1.00% or more.

In irons of the .90 to 2.00% carbon range, the pearlitic or pearlitic-cementite structures pro duced are dissimilar from regular white iron castings, such as malleable iron as cast, and by precipitating 'a percentage of the graphite in accordance with my invention there results a lowered cementite and a higher. pearlitic ratio. For example I Pcarlite Cementito Percent Percent 1.50% earbon= 88. 11. 44 less .24 carbon precipitated as nodular temper carbon leaves.

' 1.26% remaining combined carbon as. 92. 41 7; 59

This alteration produces a higher pearlitic structure in the cast state and the castings are following may be used in combination with the silicon:

. Percent Titanium .10 to -.25 Nickel .50 to 3.00 Aluminum .10 to 1.00

In addition carbide forming elements, such as manganese, molybdenum, chromium, vanadium,

etc. may be used in various combinations. The subject matter of such alloys is reserved for another application.

From the foregoing it is apparent that I have produced a new iron of the .90 to 2.00% carbon range characterized by :the presence of fine temper carbon nodules of rounded or elongated form which are from one-third to one-sixth the size of temper carbon produced by annealing 'or spheroidizing malleable iron and which are substantially unaltered by subsequent air, water 01 oil quenchings but may be substantially altered by subsequent furnace coolings. Further, it will be seen that the present invention provides a process for producing fine grained irons within,

the .90 to 2.00% carbon range during the solidification of the metal, this fine grain structure, as well as freedom from large dendritic patterns, resulting from the mixing of low temperature high carbon iron with high temperature low carbon straight or alloyed steel, the resulting pouring. temperature, which is lower than the mean temperature or the two irons before mixing, being so low, that the size of the grains are from one-tenth to one-twentieth the size' of the-usual ingotgrain produced by conventional furnace methods of producing iron and steel. 7

I claim as myinvention:

1. The method of producing high tensile The percentage of graphite so strength hypereutectoid iron castings having a mixing said molten iron with a partially finished steel having a carbon content of from .10 to 1.50% and at a temperature of from 2800 to' 3200 F., said molten iron and steel havingan endothermic reaction to provide a temperature of said mixture of about 2750 F. and pouring the mixed metals into molds to produce a cast product characterized by small grain structure and freedom from pronounced dendritic patterns.

2. The method of producing high tensile strength hypereutectoid iron castings having a,

carbon content of from .90 to 2.00% which consists in preparing by remelting a, molten iron having a carbon content of from 2.00 to 3.5% and at a temperature of from 2300 to 2'800 F., mixing said molten iron with a partially finished steel having a carbon content of from .10 to 1.50% and at a temperature of from 2800 to 3200 F. to provide a mixture having a temperature of about. 2750 F. and immediately cooling and solidifying the mixed metals to produce a solidified product characterized by small grain structure and freedom from pronounced dendrltio patterns.

3. The method of producing high tensile strength iron castings which consists in preparing molten iron having a carbon content of from .90 to 2.00% and at a temperature of about 2750 F., introducing at least two graphitizing agents and permitting the metal to solidify, said graphitizing agents precipitating temper carbon out of solution, during solidification and subsequent cooling, in the form of minute, uniformly distributed nodules.

4. The method of producing high tensile strength iron castings which consists in preparing iron having a carbon content of from .90 to 2.00% and at a temperature of about 2750 F., introducing from .85 to 5.00% of at least two graphitizing agents one of which is silicon and permitting the metal to solidify, said graphitizing agents precipitating in excess of .02% temper carbon out of solution in the form of minute, uni

formly distributed nodules.

5. The method of producing high tensile strength hypereutectoid iron castings which con.- sists in combining molten iron at a temperature of about 2750 F. and having a total carbon content of .90 to 2.00% with silicon and at least one other graphitizing agent selected from a group carbon content of from .90 to 2.00% which consists in preparing by remelting a molten high carbon iron at a low temperature, mixing said molten high carbon iron with a partially finished steel at high temperature, said molten iron and steel having an endothermic reaction to provide a temperature of said mixture of about 2750 F., introducing from .85 to 5.00% of at least two graphitizing agents into said mixed metals to precipitate in excess of .02% temper carbon out of solution in the form of minute, uniformly distributed nodules and pouring the mixed metals into molds, the cast product being characterized by small grain structure and freedom from pronounced dendritic patterns.

7. The method of producing high tensile strength hypereutectoid iron castings having a total carbon content of from .90 to 2.00% which comprises preparing molten iron having a total carbon content of from 2.00 to 3.5% and having a temperature of from 2300 to 2800 F., mixing said molten iron with a molten steel having a total carbon content of from .10 to 1.50% and having a temperature of from 2800 to 3200 F., said molten iron and steel having an endothermic reaction to provide a temperature of said mixture of about 2750 and pouring the mixture into molds to produce a cast product characterized by small grain structure and freedom from pronounced dendritic patterns.-

' JAMES L. GIBNEY. 

